The present invention relates generally to reticle and integrated circuit design and fabrication systems. More specifically, the invention relates to mechanisms for inspecting reticles and integrated circuits.
Generation of reticles and subsequent optical inspection of such reticles have become standard steps in the production of semiconductors. Initially, circuit designers provide circuit pattern data, which describes a particular integrated circuit (IC) design, to a reticle production system, or reticle writer. The circuit pattern data is typically in the form of a representational layout of the physical layers of the fabricated IC device. The representational layout typically includes a representational layer for each physical layer of the IC device (e.g., gate oxide, polysilicon, metallization, etc.), wherein each representational layer is composed of a plurality of polygons that define a layer""s patterning of the particular IC device.
The reticle writer uses the circuit pattern data to write (e.g., typically, an electron beam writer or laser scanner is used to expose a reticle pattern) a plurality of reticles that will later be used to fabricate the particular IC design. A reticle inspection system may then inspect the reticle for defects that may have occurred during the production of the reticles.
A reticle or photomask is an optical element containing transparent and opaque, and/or semi-transparent and phase shifting regions which together define the pattern of coplanar features in an electronic device such as an integrated circuit. Reticles are used during photolithography to define specified regions of a semiconductor wafer for etching, ion implantation, or other fabrication processes. For many modern integrated circuit designs, an optical reticle""s features are between about 1 and about 5 times larger than the corresponding features on the wafer. For other exposure systems (e.g., x-ray, e-beam, and extreme ultraviolet) a similar range of reduction ratios also apply.
Optical reticles are typically made from a transparent medium such as a borosilicate glass or quartz plate on which is deposited on an opaque and/or semi-opaque layer of chromium or other suitable material. However, other mask technologies are employed for e-beam exposure (e.g., stencil masks), x-ray exposure (e.g., absorber masks), etc. The reticle pattern may be created by a laser or an e-beam direct write technique, for example, both of which are widely used in the art.
After fabrication of each reticle or group of reticles, each reticle is typically inspected by illuminating it with light emanating from a controlled illuminator. An optical image of the reticle is constructed based on the portion of the light reflected, transmitted, or otherwise directed to a light sensor. Another way to inspect a reticle is accomplished by directing a beam of electrons from a scanning electron microscope towards the reticle. An image of the reticle is constructed based on secondary and backscattered electrons emitted from the reticle in response to the beam of electron towards a detector. Such inspection techniques and apparatus are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif.
During a conventional inspection process, a target image of a test structure on the reticle is typically compared to a reference image. The reference image is either generated from the circuit pattern data or from an adjacent die on the reticle itself. Either way, the target image features are analyzed and compared with corresponding features of the reference image. Each feature difference is then compared against a threshold value. If the target image feature varies from the reference feature by more than the predetermined threshold, a defect is defined. Similar techniques may be utilized to inspect an integrated circuit (IC).
Unfortunately, these inspection mechanisms frequently result in false defects when the target images significantly differ from their corresponding reference images and these differences are not caused by xe2x80x9crealxe2x80x9d defects. Several different effects may contribute to the target images differing from the reference images. When the target structures are fabricated from a set of original design patterns which are also used as the reference images, the fabrication process itself introduces discrepancies between the target structures and the reference images. By way of examples, the target structures may become tapered and reduced in size. Target structure corners may also become rounded and/or warped, while edges may become roughened. Imaging of the target structures during inspection also introduces discrepancies between the reference and target images. The imaging process may result in edge enhancement, scan persistence, and contrast changes between the substrate and material, as compared with the design patterns (or reference images). Edges may also appear thickened and charge shadowing may occur.
One technique for reducing the number of false defects includes altering the reference image until it begins to look like the target image. Several parameters of the reference image are typically changed in an iterative process until the reference image resembles the target image. This iterative process is typically complex and time consuming. Although this iterative process may result in a reference image that resembles the target image, it may also unintentionally add a feature to the reference image that corresponds to a xe2x80x9crealxe2x80x9d defect in the target image. Since a defect is defined as a difference between the reference image and the target image and the reference image has been alter to resemble the target image, this technique may cause xe2x80x9crealxe2x80x9d defects to not be found.
Accordingly, there is a need for improved mechanisms for providing reference images for inspecting corresponding target images so that real defects may be reliable found, while filtering out false defects. Additionally, there is a need for reduction in the discrepancies between the reference and corresponding target images caused by process and/or imaging effects resulting from fabricating and imaging of the corresponding test structures.
Accordingly, mechanisms are provided for generating reference images to reduce the number of false defects found when comparing a reference image to a corresponding target image of a reticle or mask (or integrated circuit). In one specific implementation, parameters that characterize the reticle making process are collected from a representative reticle or reticle. (The terms xe2x80x9creticlexe2x80x9d and xe2x80x9cmaskxe2x80x9d are used herein interchangeably). These parameters are collected (e.g., measured) from a reticle prior to inspection of the reticle. For example, a scanning electron microscope is used to measure and collect parameters from a fabricated reticle that characterize the fabrication process. The collected parameters are then used to simulate process effects on the reference images of the design data that is used to fabricate the reticles for an integrated circuit. Any suitable parameters may be collected from the reticle so as to sufficiently characterize the reticle making process so that these parameters may be used to simulate the effects of the reticle making process on the layout design data or reference representations so that the number of detected xe2x80x9cfalsexe2x80x9d defects is reduced. After the layout data (or reference representations) is altered to simulate process effects and represent a xe2x80x9crealxe2x80x9d layout after it has been fabricated into a mask, this xe2x80x9crealxe2x80x9d layout data is then altered again to simulate imaging effects. The resulting reference images now include both processing and imaging effects and may then be used during an inspection of a corresponding reticle or integrated circuit. These techniques may also be used for inspecting an integrated circuit.
The techniques of the present invention provide a reference image that results in significantly less xe2x80x9cfalsexe2x80x9d defects during inspection of a corresponding target image than a conventional produced reference image. In specific embodiments, the techniques of the present invention also provide a clear separation between effects resulting from the fabrication process and the inspection process. Thus, the effects from new techniques for fabrication or inspection can be more quickly and efficiently adapted into the separate techniques for generating a reference image.
In one embodiment, a method of generating a reference image for an inspection tool is disclosed. A plurality of parameters that characterize a process for fabricating a sample are collected. A first reference representation that is designed to be used to fabricate a target structure on the sample is provided. The first reference representation is altered based on the collected parameters so as to produce a second reference representation that simulates process effects from fabricating the sample sufficiently well so as to reduce detection of false defects during a subsequent inspection of the sample using the first reference representation. The second reference representation is altered so as to simulate imaging effect from imaging the second reference representation with the inspection tool. The altering of the first and second reference representations to simulate process and imaging effects, respectively, results in a simulated reference image that may be compared against the target structure to determine whether the target structure has any defects.
In a specific implementation, the first reference representation is altered to simulate process effects by inputting the first reference representation and the collected parameters that characterize a process for fabricating a sample into a process simulation tool which thereafter simulates the process effects by altering the first reference representation. In a further implementation, the target structure is imaged to obtain a target image. The simulated reference image is compared to the target image. When there is a difference between the simulated reference image and the target image, it is determined that there is a defect present within the target structure. In one aspect, the collection of the parameters is performed independently of the comparison. In a specific aspect, collection of the parameters is performed with a scanning electron microscope and the imaging of the target structure is performed with an optical inspection tool.
In another aspect, it is determined whether the collected parameters are within specification. An inspection is performed when the collected parameters are within specification. However, an inspection is not performed when the collected parameters are not within specification. In another embodiment, the process for fabricating the sample is adjusted when the collected parameters are not within specification.
In one aspect, the process effects include feature tapering, corner rounding, and feature shrinkage. In another aspect, the process effects further include feature warping and edge roughening. In one implementation, the imaging effects are based on one or more imaging parameters selected from a group consisting of numerical aperture of the inspection tool""s lens, magnification of the inspection tool""s lens, and coherence of the inspection tool""s illumination.
In a specific embodiment, the inspection tool is an optical inspection tool designed to measure and/or inspect samples. In one aspect, the sample is a reticle or an integrated circuit, and the target structure is an integrated circuit structure, a resist pattern structure utilizable for fabricating an integrated circuit structure, a reticle structure, or a resist pattern structure utilizable for fabricating a reticle structure.
In another aspect, the invention pertains to a inspection system operable to generate a reference image for an inspection tool. The inspection system includes one or more processors and one or more memory. At least one of the processors and memory are adapted to perform one or more of the above described inventive procedures. In one embodiment, the computer system is in the form of an inspection system. In yet another aspect, the invention pertains to a computer program product for generating a reference image for an inspection tool. The computer program product includes at least one computer readable medium and computer program instructions stored within the at least one computer readable product configured to cause a processing device to perform one or more of the above described inventive procedures.